Screw compressors are a type of rotary volume compressor and thanks to their reliability, long lifetime and versatility, they are widely used in a lot of different industries. Compared to piston compressors, they are overall more efficient and flexible.
First of all, let us take a look at the screw compressor element. The element consists of two rotors that look like helixes, each with a different shape and number of ‘grooves’/‘teeth’ (usually one with 4 and one with 6 teeth. To put it simply, atmospheric air is sucked into this screw element and as the air progresses along the rotors, it gets compressed.
The screw element (block) consists of two rotors that are similar to a helix with a large climb with a different number of teeth. The most usual ratio of teeth is usually 4:6. As the air progresses along the rotors the air is compressed as the volume space between the rotor teeth decreases in the direction from the suction to the outlet.
The most widespread type of screw compressors are lubricated compressors with oil injection. For sensitive processes regarding the quality of compressed air non-lubricated compressors are used.
The compression process, in case of a lubricated compressor, proceeds as follows:
The air circuit:
Air is drawn through the filter and an open inlet valve, into the compressor element and gets compressed. A mix of compressed air and oil flows into the air receiver/oil separator via a check valve. The air is discharged through an outlet valve via a minimum pressure valve and the air cooler.
During loaded operation, a minimum pressure valve keeps the pressure in the separator tank above a minimum value, required for lubrication. An integrated check valve prevents the compressed air downstream the valve from being vented into the atmosphere during unloaded operation. When the compressor is stopped, the check valve and inlet valve close, preventing compressed air (and oil) to be vented into the air filter.
The oil circuit:
In the air receiver/oil separator, most of the oil is removed from the air/oil mixture by centrifugal action. The remaining oil is removed by the oil separator. The oil is collected in the lower part of air receiver/oil separator, which serves as an oil tank.
The oil system is provided with a thermostatic bypass valve. When the oil temperature is below its set point, the bypass valve shuts off the supply to the oil cooler and the oil cooler is bypassed. Air pressure forces the oil from the air receiver/oil separator through the oil filter and oil stop valve into the compressor element.
The bypass valve starts opening the supply from the cooler when the oil temperature has increased to the set point. At approx. 15 ˚C (27 ˚F) above the set point, all the oil flows through the oil cooler. An oil stop valve prevents the compressor element from flooding with oil when the compressor stops.